In conventional mineral insulated wall cables, the closure is required to provide a suitable seal against the outer environment and prevent ingress of materials and moisture. Dual-wall mineral insulated cables require the same closure or sealing of the outer sheathing or outer wall for the same reasons. In addition, the inner sheath or inner wall, which is made from a different material, is also required to be sealed to stop migration of elements and materials from the outer sheath or outer wall to the thermocouple wires. The new methods are required to maintain the integrity of both the inner and outer sheaths or inner and outer walls of the dual-walled cable design. As the inner and outer sheaths or inner and outer walls are different materials, they ideally require closure separately with, if possible, no mixing of the sheathing materials during welding.
The present invention solves the problem of ensuring both walls or sheathing material of a dual-wall mineral insulated cable are sealed or closed. When using the same methods as used with conventional mineral insulated dual wall cables, in operation and at temperature, the weld closures show faults and splitting or blowing open and exposing the inner MgO powder and breaking the seal. The splitting or blowing of the sheath closure weld is due to the mixing of the inner and outer sheathing materials during the welding process giving a weld or outer sheathing area of significantly different alloy, to that of the rest of the cable. Sealing the inner wall or the inner sheath will stop/reduce migration of elements from the outer wall or outer sheath. Sealing the outer wall or outer sheath will provide a continuous protection from the environment. Both sealing the inner wall and outer wall are important for the termination/closure of the sheathing at a hot junction area of a thermocouple cable.